For certain undergrad classes, I assign a take-home exam, rather than the more familiar in-class exam. When I distribute the exam, I remind the students that this is an individual exam, and that they may not work in groups, and then I add "Believe me, I can tell when you cheat". This, however, is a bluff. Unless it is superobvious, I can't tell if students work together; I only tell them I can to scare them into honoring the rules.
Are there any ways, other than lying to the students, to prevent (or at least minimize) this type of cheating in take-home exams?